A current trend in large scale computers is to interconnect for operation a large number of small processors in parallel to achieve a system that has the computing power of approximately the total powers of the interconnected processors.
In such systems, it is advantageous that the network used to effect the interconnections operates with high speed. It is also advantageous that the network be reliable and be scalable so that the number of interconnected processors can be readily increased when the total computing power needs to be increased.
The advantages of optics for use in such interconnection networks has long been recognized and a variety of optical interconnect techniques have been proposed.
Of particular relevance to the instant invention have been schemes that use optics to implement "Clos" networks. Clos structures were early described in a paper entitled "A study of non-blocking switching networks" by C. Clos in the Bell Systems Technical Journal Vol 32 pp 406-424 (1953). In a Clos network, or switch, the switching function is achieved by several, typically three or five, separate interconnected serial networks. Schemes that use a Clos network involving optics are also known.
A first such scheme has been described by S. It. Lin, T. P. Krile, and J. F. Walkup in a paper entitled "Two-Dimensional Optical Clos Interconnection Network and its uses", Applied Optics, Vol. 27, No. 9, pp. 1734-1741, May 1, 1988, that describes a 2-D optical three-stage Clos interconnection network made up of a number of feasible crossbars switches of medium size. The optical implementation uses LCLVs (Liquid Crystal Light Valves) to implement each stage of a 3-stage Clos network. Assuming theft all the switches are of the same size (n.times.n where n=.sqroot.N for an N.times.N Clos network), then one stage of the 3-stage Clos network will have the following: an input array of N.times.N, which is replicated .sqroot.N.times..sqroot.N times, an LCLV with N.sqroot.N.times.N.sqroot.N channels which is used to mask out appropriate inputs (out of the replicated input array). Then a lenslet array of N.times.N elements is used to collect the desired intermediate output elements. The outputs of one stage are then connected as inputs to the next stage.
A paper by Golshan and Bedi entitled "Optical Implementation of Clos Network Using Reversible Nonlinear Interface Devices" Proc. of 33rd Midwest Symposium on Circuits and Systems, Vol. 2 pp. 915-917, August 1990, describes another optical Clos network constructed from reversible nonlinear interface (RNI) devices. The proposed building block is a 2.times.2 optical switch box capable of performing the 4 basic switching actions (direct, cross, up and down broadcast). The 2.times.2 switch uses reversible nonlinear interface 4 RNI devices. The propagation time for an RNI device varies with the type of materials used. In the paper, an example implementing a 4.times.4 crossbar switch is given, with a total delay of 16 picoseconds to 80 nanoseconds, depending on the basic RNI implementation. In general, to implement a crossbar of N.times.N, there is a need for N.sup.2 RNI devices and a propagation time (neglecting the control problem) of log.sub.2 (N) times the propagation delay for one RNI device. Using such crossbar switches, it is possible to make an optical Clos network.
Neither of these schemes appears to have found wide acceptance for various reasons. Typically, they do not scale readily to large networks, such as ones involving thousands of elements to be interconnected. Also, they tend to be slow despite the use of optics because the techniques used to switch the optics is slow. Additionally, such schemes generally have involved multiple switching layers which makes for added complexity.